1. Field of the Invention
This invention relates to an apparatus and method for determining characteristics of a wavefront. More particularly, the invention relates to an apparatus and method for increasing the dynamic range of a wavefront sensor in measuring a tilt angle and a degree of focus of the wavefront.
2. Description of the Related Art
Wavefront sensors have been used, for example, in camera focusing technology to measure the distance between an object and a device, e.g., the camera, by sending out a signal wavefront and to measure the round trip time of such signal wavefront. Knowing the distance, the camera can focus properly. Particularly, wavefront sensors include elements which provide information about phase distortions or aberrations in a received wavefront, and which analyze, measure, and provide information signals to correct for the aberrations received in optical wavefronts. These phase distortions are produced in an image by many causes, such as a thermal expansion or contraction of the device due to varying temperatures, which changes distances between lenses and/or mirrors therein.
In the laser art, wavefront sensors, such as Hartman sensors, have been used to measure the wave front quality of a laser beam. In measuring the wave front quality of a laser beam, two major components of measurement are determination of a tilt angle and determination of a degree of focus of the incoming wavefront. Tilt angle is the local slope of a segmented wavefront over the dynamic range of the wavefront sensor. The degree of focus is the low order aberration of the aberration polynomials representing the local curvature of the segmented wavefront over the dynamic range of the wavefront sensor. Dynamic range is the operating range of the wavefront sensor generally measured, in xcexcm. The wavefront of incoming beam is defined as a surface that is normal to the local propagation direction of the beam. Wave-aberration polynomial represents the departure of the actual wavefront from a perfect spherical reference surface.
A Hartman sensor generally employs an array of lenses to focus an incoming laser beam to a set of focal points. An array of optical detectors detects the focal points and transmits an output to a measuring unit. The measuring unit compares the light intensity at various focal points on the detector with a reference beam or with a set of nominal values. Based on the readings of the measuring unit, an adaptive optical system then corrects a tilt angle and a degree of focus in the optical wavefront. The sensor also compensates for errors or deviations within the laser or within the atmosphere through which the laser beam travels.
FIG. 1 illustrates a perfectly straight, non-distorted, wavefront 120 passing through a multiple lens array 140 (an array of six lenses are shown for exemplary purposes), which have a uniform focal length. Multiple lens array 140 splits incoming wavefront 120 into a series of subapertures, each of which creates a focal point (one of P1-P6) on a detector 130. The wavefront tilt angle is determined by the position of each focal point P with respect to the corresponding reference focal points. The wavefront itself can be reconstructed by integrating the wavefront tilt angle over the range of lens array 140.
Detector 130 can be a CCD camera which is assigned to a multiple fields (six in this example) corresponding to the number of lenses in lens array 130 to detect where focal points are and to measure the local tilt angle with respect to nominal or reference focal points. Focal points P1-P6 of the perfectly straight, non-distorted wavefront 120, as illustrated in FIG. 1, are nominal or reference focal points whereby tilt angle is zero.
In a practical world, however, incoming wavefront 120 is oftentimes distorted, represented by a curved line or inverted xe2x80x9cSxe2x80x9d curve shown exaggerated in FIG. 2 for illustration purposes. Lens array 140 splits incoming wavefront 120 into a series of subapertures, each of which creates a focal point (one of Pxe2x80x21-Pxe2x80x26) on detector 130. Distorted wavefront 120 has a wide collection of propagation directions and lens array 140 focuses wavefront 120 into different positions of focal points Pxe2x80x21-Pxe2x80x26 on detector 130. For example, focal points Pxe2x80x21 and Pxe2x80x26 are above nominal focal points P1 and P6, respectively, because the segment of wavefront 120 in front of lenses L1 and L6 have a negative slope, are slanted to the left, or having a backward slash configuration ( ). Conversely, focal points Pxe2x80x23 and Pxe2x80x24 are below nominal focal points P3 and P4, respectively, because the segment of wavefront 120 in front of lenses L3 and L4 have a positive slope, are slanted to the right, or having a forward slash configuration ( ). The more negative the slope is, the further upward the focal point falls from the corresponding nominal focal point on the detector. Similarly, the more positive the slope is, the further downward the focal points falls from the corresponding nominal focal point on the detector.
The conventional wavefront sensor described above poses a problem that when the incoming wavefront is overly distorted, such that the local focal point is either too high or too low from the nominal or reference focal point, the focal point misses the local detecting field and is detected by a neighboring detecting field of the detector. The detector assigned to measure focal point of the local detecting field is confused with the measurement readings of the neighboring detecting fields. Therefore, when incoming wavefront is distorted beyond the dynamic range of the detector, the measurement reading of the detector becomes meaningless.
In light of the foregoing, there is a need for a wavefront sensor which has a large dynamic range to enable an easy and accurate measurement of the tilt angle and the degree of focus of incoming wavefront. In addition, the wavefront sensor needs to have a compact design and be insensitive to external disturbances.
The advantages and purposes of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The advantages and purposes of the invention will be realized and attained by the elements and combinations particularly pointed out in the appended claims.
To attain the advantages and in accordance with the purposes of the invention, as embodied and broadly described herein, one aspect of the invention is an apparatus for determining characteristics of an incoming energy beam. The apparatus comprises a multi-lens array, a screen, and a beam detector. The multi-lens array focuses the energy beam to a multiple focal points. The screen, positioned adjacent to the multi-lens array, has at least one aperture to allow a portion of the energy beam to pass. Each aperture is aligned with a lens of the multi-lens array. The screen blocks the remainder of the energy beam from arriving at the multi-lens array. The beam detector detects at least one focal point of the energy beam passing through the corresponding at least one aperture and determines the characteristics of the passing energy beam with respect of a reference beam.
Another aspect of the invention is an apparatus for determining characteristics of an incoming energy beam including a multi-lens array and a beam detector. The multi-lens array focuses the energy beam to a multiple focal points. The beam detector detects the focal points and determines the characteristics of the passing energy beam with respect of a reference beam. The apparatus comprises a screen positioned adjacent to the multi-lens array. The screen has at least one aperture to allow a portion of the energy beam to pass. Each aperture is aligned with a lens of the multi-lens array. The screen blocks the remainder of the energy beam from arriving at the multi-lens array.
Yet another aspect of the invention is a method for determining characteristics of an incoming energy beam. The method comprises the steps of providing a screen having at least one aperture, passing a portion of the energy beam through the at least one aperture to arrive at and be focused by corresponding at least one lens of a multi-lens array, blocking the remainder of the energy beam from arriving at the multi-lens array; and determining characteristics of the focused energy beam with respect to a reference beam.
A further aspect of the invention is a method for making an apparatus to determine characteristics of an incoming energy beam. The method comprises the steps of providing a screen and providing a beam detector. The screen is for allowing a portion of the energy beam to pass through at least one aperture on the screen and arrive at corresponding at least one lens of a multi-lens array. The screen blocks the remainder of the energy beam from arriving at the multi-lens array. The multi-lens array focuses the passing energy beam to corresponding at least one focal point. The detector is for detecting the at least one focal point to determine the characteristics of the passing energy beam.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. Additional advantages will be set forth in the description which follows, and in part will be understood from the description, or may be learned by practice of the invention. The advantages and purposes may be obtained by means of the combinations set forth in the attached claims.